The present invention relates to a device for detecting the oxygen density of room air or the like by detecting an ion current in a combustor.
The oxygen density of the air in the room where a heating apparatus or a water heater with an open type combustor using indoor air for combustion is installed is an essential factor for safety and health. Therefore, it is necessary to detect the oxygen density of the air in such a room and to stop the combustor or to ventilate the room if necessary.
As shown in FIG. 1, the ion current value of flames in an open type combustor decreases with the oxygen density of air in the room. Therefore, the oxygen density of air in the room can be detected by detecting the flame ion current. For instance, the device may be constructed such that a reference level is suitably set and the reference level is compared with each of the ion current levels which are provided successively. When the ion current level is lower than the reference level, an output signal is generated to detect the oxygen density of the air in the room.
However, if the same reference value is used for more than one open type combustor, the oxygen density cannot be detected with a high accuracy because the ion currents of the open type combustors are not always equal to one another due to variations in characteristics which occur during manufacture or changes with time. Therefore, the relationships between ion current and oxygen density of the combustors are different.
This will become more apparent from the following description. It is assumed that first and second open type combustors A and B have different ion current characteristics with time as shown in FIG. 2. If, in this case, the common reference level for the combustors A and B is set to the level a indicated in FIG. 2, while in the combustor A an oxygen density of about 18% is detected, in the combustor B the oxygen density about 18.5%. Thus, the oxygen density cannot be accurately detected with plural combustors if a single reference level is used.
If the reference level for the first open type combustor A is set to the level a and the reference level for the second combustor B is set to the level b as shown in FIG. 2, then the oxygen density of the room air can be detected with a high accuracy. Therefore, a method may be advantageously used in which characteristics of ion current level with time are obtained for a plurality of open type combustors by prior tests, and reference levels set for each combustor separately.
However, that method still involves drawbacks in that it is cumbersome and expensive to test a large number of manufactured open type combustors one by one to obtain the individual reference values.